The goal of the proposed research is to define the underlying mechanisms that control the postnatal critical period for cortical development in kittens. The cat visual system is well suited for the analysis of cortical plasticity as recent studies indicate that environmental input not only directs on-going development, but also plays a role in activating the critical period itself. Specifically, rearing in total darkness prolongs the critical period in cats well beyond the normal age limits. We believe that this preparation provides an ideal experimental model, when compared with age matched normals, for isolating the neurological factors that correlate with functional plasticity. We will compare normal and various types of deprived kittens in studies directed at the following specific aims: 1) To define the role of environmental input in the onset and duration of the critical period: Electrophysiological studies will directly determine how the elimination of input, and the delayed introduction of input alter the normal time course of cortical plasticity. 2) To define the relationship between anatomical and electrophysiological plasticity: Trans-synaptic autoradiography and electrophysiological recordings will be used together to specify how the environment effects afferent input to the cortex. 3) To identify underlying factors that correlate with the developmental state of physiological plasticity. One approach will be to examine the effect of environmental input on the development of specific neurotransmitter systems. A combination of receptor binding, immunohistochemistry and biochemistry will be used to provide a broad range analysis of transmitter systems. A second approach will be to examine whether visual input can affect gene expression. The kitten cortex serves as a well studied and easily manipulated neurological model. Our proposed multidisciplinary approach directly addresses the ways in which the environment controls the physiology, anatomy and biochemistry of the developing nervous system.